Abstract : The report describes an FAA-sponsored development and flight test program of a directional glide-slope system that is fully compatible with ICAO ILS standards and conventional airborne receivers, but that does not rely upon the ground-reflected signals required by conventional glide slope systems. The theory and application to ILS of the rugged, simple, 20-wavelength, slotted-waveguide array is described in detail from its original design through X- and S-band model tests and its final full-scale fabrication to flight test at three different problem sites. (Author)